I had the same problem with my RX580 8GB using 18.8.1. Usually its default memory frequency is 2000 MHz but it kept dropping down to 1750 MHz. I also had some stability issues with Windows 10 blue screening a couple of times. I ended up going back to 18.5.1 and haven't had any problems since. 18.8.1 seems like a very buggy release.

OK，so if I have some problem with the graphics card driver next time, thie place is unuseful for working out，right？ I will tell AMD how do I want to say something so hard even I will use the translate,hehe, anyway I have turn back to the 18.7.1, but I feel despair,really